(Reuters) - U.S. farmers will likely use less nitrogen fertilizer this season with the cost sky-high even though the price ofnatural gas, the key ingredient to make it, is down 40 percent from last year.

The reduction in usage should hit corn plantings more than other crops, since nitrogen is the key booster of corn yields.

"What we're seeing this season is a reduction in rates," said Ray Carpenter, senior vice president of agronomy for Farmers Cooperative in Ames, Iowa, referring to nitrogen bookings. "Reduced rates mean reduced yield."

Fertilizer's base feedstock,natural gas, is around $2.716 per million BTU, down from $4.536 last year. The price of anhydrous ammonia - a popular nitrogen fertilizer - remains high, around $650-$700/ton.

The disparity between the steep cost of fertilizer and the lower cost ofnatural gasis because fertilizer inventories remain thin due to 2014 shipping backlogs and because the industry is controlled by a few big players.

"The reality is input costs don't come down as fast as the break-even price," said analyst Sterling Liddell of Rabobank.

Crops require nitrogen, potassium and phosphorus, with corn the single biggest user of nutrients. It takes roughly one pound of nitrogen per acre to yield one 56-pound (25 kg) bushel of corn.

Crop specialization has made corn acres, planted year after year, increasingly dependent on nitrogen.

"I don't want to over-spend," said Iowa farmer Mark Recker, who will begin fieldwork soon. "I'm debating how much side dressing I want to do, splitting applications. I may back off compared to previous years."

Recker, an Iowa Corn Growers director, added: "When you had $6 corn, it drove me to put on a little higher nitrogen to get higher yields. Now you look at it much more economically."

The U.S. Agriculture Department projects farm gate prices for corn this season at $3.50-$3.90 a bushel, versus $6.89 two years ago. Soybeans are projected at $9.45-$10.95, versus $14.40 in 2013.

Liddell said if farmers plant 1 million-2 million fewer corn acres this spring, anhydrous ammonia could fall $50-$100 a ton.

Some analysts are more bearish.

"The cost of gas and ammonia right now is around $100 a ton. They're selling it in Midwest for $600 a ton," said one fertilizer industry source.

"So there's a lot of room for prices to come down. You could see a 40-50 percent drop by the end of the year."